1. Field of the Invention
The invention relates in general to a lamp driving circuit, and more particularly to a lamp driving circuit applied in a backlight module.
2. Description of the Related Art
Refer to FIG. 1A and FIG. 1B. FIG. 1A is a diagram of a conventional backlight module lamp driving circuit; FIG. 1B is a circuit diagram of a conventional feedback circuit. Liquid crystal display uses a fluorescent lamp 102 in a backlight module lamp driving circuit 100 as the backlight source to provide the light source during display. A conventional backlight module lamp driving circuit comprises a feedback circuit 104, a DC-to-AC converter 106, a voltage raising unit 108 and a controller 110. The feedback circuit 104 provides a feedback signal FSi in response to driving power signal PS required to drive the fluorescent lamp 102, so that the backlight module lamp driving circuit 100 adjusts the driving power signal PS for the fluorescent lamp 102 to achieve the desired brightness and maintain stability in response to the feedback signal FSi. The conventional feedback circuit 104, which is an ordinary rectification circuit, comprises diodes D1 and D2, a resistor R and a capacitor C, rectifies and filters the AC driving power signal PS, then provides the feedback signal FSi. When the rectification circuit corresponds to a small-sized liquid crystal display, the position of disposition, as illustrated in FIG. 1, can only be coupled to the fluorescent lamp 102 and the grounding end, or between the high voltage side coil of the voltage raising unit and the grounding end. Since the single end of the fluorescent lamp 102 is connected to the ground voltage, the feedback circuit 104 is serially connected to a low-voltage node.
As the size of liquid crystal display becomes larger and larger, the length of the fluorescent lamp 102 becomes longer and longer, and so too the striking and operating voltage of the fluorescent lamp 102 become higher. When the length of the fluorescent lamp 102 is over 900 mm, the required voltage of the fluorescent lamp 102 will be over 1.5 KV. So the lamp driving circuit 100 in the backlight module of a large-sized liquid crystal display has evolved from original single side drive mode to dual side drive mode, so that the two ends of the fluorescent lamp 102 will not have a low-voltage node. However, if a conventional feedback circuit 104 is used to convert the high voltage driving power signal PS into a feedback signal Fsi, the voltage of the feedback signal Fsi will be too high thus cannot be used by the controller 110 directly. Moreover, the elements of conventional feedback circuit 104 is too weak in terms of voltage resistance, so is unable to receive the high voltage driving power signal PS. Therefore, conventional feedback circuit 104 cannot be applied in the floating system backlight module 100.